Singlet scalars as Higgs imposters at the Large Hadron Collider

TL;DR

The paper investigates whether an electroweak singlet scalar $S$ with loop-induced dimension-five couplings to vector bosons can mimic a Standard Model Higgs signal at the LHC. It shows that the singlet naturally yields a hierarchy of diboson decay widths, with production dominated by $gg$ fusion and decays into $gg$, $\gamma\gamma$, and $Z\gamma$ typically outweighing $WW$ and $ZZ$ when $m_S \lesssim 2m_W$, while production and decays remain calculable via the introduced couplings. Crucially, the $\gamma\gamma$ and $Z\gamma$ branching fractions can be enhanced by factors of order $10$–$30$ relative to the SM Higgs, enabling potentially observable resonances even for $m_S$ above the $WW$ threshold. The authors perform detailed LHC phenomenology for $\sqrt{s}=7$ and $14$ TeV, including backgrounds and selection cuts, and show that diphoton searches can provide early discovery reach for moderate enhancements, with the $Z\gamma$ channel offering complementary sensitivity.

Abstract

An electroweak singlet scalar can couple to pairs of vector bosons through loop-induced dimension five operators. Compared to a Standard Model Higgs boson, the singlet decay widths in the diphotons and Z gamma channels are generically enhanced, while decays into massive final states like WW and ZZ are kinematically disfavored. The overall event rates into gamma gamma and Z gamma can exceed the Standard Model expectations by orders of magnitude. Such a singlet may appear as a resonant signal in the gamma gamma and Z gamma channels, even with a mass above the WW kinematic threshold.

Figures (8)

• Figure 1: Enhancement in the event rate of a 115 GeV$/c^2$ Higgs boson versus the number of extra generations. Each generation includes one quark doublet and one lepton doublet.
• Figure 2: Decay branching fractions of a singlet scalar $S$ with $m_S=115$ GeV$/c^2$ into pairs of electroweak vector bosons. In the plot we assume the production rate of $S$ is the same as that of a SM Higgs with the same mass. In general the $\gamma\gamma$ mode has the largest branching fraction, followed by $Z\gamma$, $WW$, and $ZZ$ channels.
• Figure 3: Decay branching fractions of a singlet scalar $S$ into pairs vector bosons as a function of mass, for three different choices of $\kappa_W$ and $\kappa_B$, assuming a SM coupling strength to gluons. For comparison the branching fractions for a SM Higgs boson is also shown in the lower-right figure.
• Figure 4: Enhancement of $B\sigma(gg\to S\to \gamma\gamma)$ and $B\sigma(gg\to S\to Z\gamma)$ relative to the SM for $m_S=115$ GeV$/c^2$. Top two plots assume the same production rate as in the SM while the bottom plots assume 10% of the SM production. Enhancements of ${\cal O}(10 - 30)$ are seen for both the $\gamma\gamma$ and $Z\gamma$ modes.
• Figure 5: Distributions of $M_{\gamma\gamma}$ from pseudo-experiments for $M_S=115$ and 165 GeV$/c^2$ and $\sqrt s=7$ TeV for 10 fb$^{-1}$ of integrated luminosity. The background subtracted distribution more clearly shows the possible $\gamma \gamma$ peak.